Prenatal cytodifferentiation in the pars distalis of the dwarf mutant mouse.

The pars distalis of the pituitary gland was studied by means of electron microscopy in 14- to 18-day dwarf embryos (dw/dw) obtained from matings of dwarf homozygotes in which reproductivity was induced by hormone supplementation and pituitary transplants to the renal capsule. At 14 days of gestation the dwarf pars distalis shows fewer granulated cells than in the normal. Although somatotroph-like and mammotroph-like cells are identifiable in the normal pars distalis as early as 15 days, comparable cells are not present in the dwarf. By 17 and 18 days, the dwarf exhibits an ambiguous cell-type with a combination of somatotroph and mammotroph characteristics. At 18 days the dwarf also continues to show numerous nongranulated cells with large aggregates of glycogen, whereas in the normal pars distalis comparable glycogen-containing cells are relatively scarce.